As a mounting technique for mounting a semiconductor device on another semiconductor device, a printed board, a tape carrier, etc., there is a technique for forming a protrusion electrode for external connection on an electrode pad of the semiconductor device. One of the methods of forming the protrusion electrode is to use an electroless plating method for forming the protrusion electrode on the electrode pad composed or mainly composed of Al.
Compared to an electrolytic plating method, the electroless plating method can eliminate steps such as a sputtering process required for forming a barrier metal layer and for forming an electrode in a plating process, a photo process required for forming a pattern of the protrusion electrode, and an etching process required for removing a resist used in the pattern forming process and for removing the barrier metal used in the plating process. As described above, the electroless plating method has beneficial features in reducing cost and in quickening the time of delivery.
However, the conventional arrangement for forming the protrusion electrode using the electroless plating method causes a problem in the adhesion strength of the protrusion electrode to the electrode pad. For explaining this problem, the following will explain steps for forming the protrusion electrode on the electrode pad composed of Al or alloy mainly composed of Al (Al alloy) using the electroless plating method, with reference to FIGS. 9(a) through 9(e).
FIG. 9(a) shows a semiconductor device before the protrusion electrode is formed. An electrode pad 102 composed of Al or Al alloy is formed on a semiconductor substrate 101. On the electrode pad 102, a first protection film 103 is further formed so as to have an opening where the protrusion electrode is to be formed. Oxide film 104 exists over a surface of the electrode pad 102 where the first protection film 103 has the opening portion.
In a step shown in FIG. 9(b), the oxide film 104 over the surface of the electrode pad 102 is completely removed with sodium hydroxide, phosphoric acid, etc. The oxide film 104 is removed in a following reason. Namely, if the oxide film 104 exists over the surface of the electrode pad 102, a shape and reliability of the protrusion electrode are significantly affected while the protrusion electrode is formed on the electrode pad 102 using the electroless plating method. Thus, in the electroless plating method, the oxide film 104 is removed from the surface of the electrode pad 102 with sodium hydroxide or phosphoric acid as pre-treatment of plating, thereby ameliorating the shape of the protrusion electrode formed with Ni or Ni alloy.
FIG. 9(c) shows a zincate treatment, which is a pre-treatment process for depositing Ni or Ni alloy as the protrusion electrode on the electrode pad 102 using the electroless plating method. In the zincate treatment, the displacement reaction between Zn and Al or Al alloy composing the electrode pad 102 is uniformly carried out, thereby forming a Zn layer 105 on the surface of the electrode pad 102.
In a step shown in FIG. 9(d), a protrusion electrode 106 composed of Ni or Ni alloy is formed on the electrode pad 102 using the electroless plating method. In this step, a layer of Ni or Ni alloy as a core is first formed through displacement reaction with respect to Zn, and then the protrusion electrode 106 is formed through autocatalytic reaction. Thus, by uniformly forming Zn grains, the protrusion electrode formed with Ni or Ni alloy plating uniformly grows. As a result, the protrusion electrode 106 can be obtained in a small grain size and in a good shape.
As shown in FIG. 9(e), an Au layer 108 is formed on the protrusion electrode 106 composed of Ni or Ni alloy, in a displacement Au plating process. The Au layer 108 may achieve an effect of preventing the oxidation of Ni or Ni alloy composing the protrusion electrode 106.
In the step shown in FIG. 9(d), the protrusion electrode 106, which is formed through the autocatalytic reaction of Ni or Ni alloy, is also formed on the first protection film 103. At a boundary portion of the first protection film 103 and the protrusion electrode 106, however, a minute gap 107 remains without chemically bonded.
In the displacement Au plating process shown in FIG. 9(e), displacement Au plating liquid enters the minute gap 107 between the first protection film 103 and the protrusion electrode 106. Further, the displacement Au plating liquid entered into the gap 107 carries out incomplete displacement reaction between Ni and Au because the liquid is not sufficiently replaced. This causes a phenomenon that only Ni dissolves. Consequently, as shown in FIG. 9(e), the protrusion electrode 106 is thinned down at the opening portion of the first protection film 103, thereby significantly lowering the adhesion strength of the protrusion electrode 106 to the electrode pad 102.